A process for producing a denitration catalyst using a slurry containing a catalyst component, such as vanadium, has been described in, for example, Patent Documents 1 to 3.
For example, a conventional denitration catalyst is produced through the steps summarized as follows.
(1) A ceramic fiber honeycomb structure is immersed in a slurry of titania particulates suspended in a silica sol so that titania is supported on the honeycomb structure. The honeycomb structure is then removed from the slurry, dried, and calcined.
(2) The honeycomb structure processed in the above step (1) is immersed in an aqueous solution of ammonium metavanadate (hereinafter referred to as AMV). The honeycomb structure is then removed from the aqueous solution, dried, and calcined.
(3) The honeycomb structure processed in the above step (2) is immersed in an aqueous solution of ammonium metatungstate (hereinafter referred to as AMT). The honeycomb structure is then removed from the aqueous solution, dried, and calcined.
In the above-described conventional production process, titania can be supported inside the ceramic fiber sheet that forms the honeycomb structure. However, vanadium is supported on titania via the adsorption of AMV onto titania; therefore, vanadium is easily supported on titania near the surface of the ceramic fiber sheet, but is hardly supported inside the ceramic fiber sheet. Accordingly, vanadium tends to be supported only on titania existing near the surface of the ceramic fiber sheet.
Catalytic performance is not exhibited by titania alone, but is exhibited only when vanadium is supported on titania. Therefore, in a catalyst produced by the conventional process, only the surface area of the ceramic fiber sheet functions as a catalyst. Generally, in a denitration reaction, the gas migration on a film of laminar flow is rate-determining step for the reaction. Therefore, use of such a catalyst produced by the conventional process, where only the surface area functions as a catalyst, does not affect the ordinary denitration performance.
However, an exhaust gas from a coal-fired boiler, for example, contains some substances that are catalyst-poisoning components. As such poisoning components adhere to the catalyst with time, the denitration performance of the catalyst is gradually degraded. In such a case, if only the surface area of the ceramic fiber sheet functions as a catalyst, the poisoning component will adhere to the catalyst surface within a short period of time. As a result, there will be a significant degradation in catalytic performance, shortening the life of the catalyst.
In addition, in the conventional production process, a three-step production process including immersion, drying, and calcination is required for each component to be supported. The production thus involves multiple steps, and, in particular, the calcination step is often rate-determining step for the production. Accordingly, the productivity is poor, and this leads to high cost.    Patent Document 1: JP-A-2003-326167    Patent Document 2: JP-A-2008-024565    Patent Document 3: JP-A-2007-330856